Jacob Lateiner, the great American pianist and teacher, used to tell his students that they should write music every day. If not their own, then they should copy out someone else's. He felt that the physical act of a hand guiding a scratching pencil across paper was a vital part of a person's musical life, and that it would naturally lead to their fingers depressing the keys with more intelligence and care.

In the days before photocopying, such a task was a common necessity, and composers might often make a copy of an admired work by a colleague. In those same days, and right up to the years following the first world war, almost everyone who performed also composed. The idea that one could exist as a solo instrumentalist simply by playing the music written by others was unheard of. To perform was necessarily to introduce one's own work (usually recently written) to the public. There are examples of composers writing for others of course, (Schubert for Hummel or Schumann for Liszt, for instance), but a pianist with an empty portfolio would have been looked upon with as much incredulity as a chef without an original recipe.

It all began, I think, in the early 1830s with Chopin's Twelve Etudes Op 10, dedicated to Liszt. The first etude's extended C major arpeggios, sprinting like a graceful leopard from the top to the bottom of the keyboard, were, in a way, a template for all that would follow, and the virtuoso soloist's first "costume". It is as if we could now see the actual shape of the composer's hand on the keyboard, his musical inspiration incarnate in the very flexing of the fingers (the span of a tenth rather than the conventional octave). The instrument, too, was extending and developing by the decade, until the piano became the romantic instrument par excellence. Its combination of power and self-sufficiency (augmented by a steady flow of great repertoire) made it the ideal stage-prop for the newly invented "recital" - a word coined by Liszt emphasising the dramatic nature of his all-piano evenings, when the hero would step on to the stage and sit in front of his wooden "steed" - the drama begun before the first note was struck. Though the violin could be cuddled to the chin and become part of the performer's body, only the piano could embrace and reach out even to the cloakrooms of the velvet boxes.

It is ironic that this greatest pianist-composer of all, Franz Liszt, should have unwittingly begun the process of change from creator-performers to mere solo instrumentalists by breaking the ground on which new, bourgeois, musical middle-men could build their recreative empires. This change became the dominant style as 19th-century self-determination gradually turned into 20th-century self-consciousness: "What if I can't write a masterpiece?" The Victorians had no such qualms, and every pianist published their own compositions. These were often small genre pieces in which, like visiting cards, the players' identity could be discerned. Leopold Godowsky had a superb contrapuntal technique: his pieces twist and turn with inner voices. Moriz Rosenthal had an uncanny way with feather-light double notes, and he seldom failed to decorate his works with such delicate lace.

Most pianists also attempted large-scale works, with varying degrees of success. The piano concerto was the obvious spin-off, and Hyperion Records has been exploring the obscurer corners of this territory for two fruitful decades now, with concertos by earlier lions of the keyboard, such as Adolph von Henselt, Emil von Sauer and Moritz Moszkowski, now recorded and regularly heard on the radio, if not yet in the concert hall. But Ignaz Paderewski, Eugen d'Albert and Anton Rubinstein also wrote respectable and much-performed operas; Ignaz Friedman and Nicolai Medtner wrote excellent quintets, and others - Saint-Saëns, Rachmaninov, Busoni and Prokofiev - while among the greatest instrumentalists of their day, are now actually known more for their compositions.

By the end of the second world war, with a few exceptions, composers no longer played their own instrumental compositions, and few instrumentalists wrote. The ideal of specialisation, with its closed shop and the resulting displacement between creator and performer, had become the norm.

But there is a modesty that is snobbery and one that is laziness, but a much more common form is just plain timidity. Anyone who can read music can write it too - and should. It doesn't have to be performed, and it may not be very inspired, but to be totally divorced from the act of creation risks making us neighbours rather than relatives to the works we play. And, by the same token, composers who never perform risk writing music that is impractical and even unplayable. (It should be pointed out that the worlds of jazz and of the church organist are two areas where creating and performing have always been indivisible.)

But there is a further issue to consider. Music itself has no common language any more, no universally accepted style. It is one thing to make a two-piece suit from a pattern, but quite another challenge to invent a whole new way of dressing. Bach, Mozart, Beethoven and Brahms each had harmonic and structural templates with which they could begin to work. These templates today have been consigned to museums; we arrive at our contemporary studios with workbenches bare, but with a bewildering array of materials from which to choose. It is no wonder that it all seems rather overwhelming.

However, I want to encourage, not accuse. Words are a good place to start composing music. Not only is a text its own structure in some ways, but the human voice has certain finite limits - and all of us can sing. Variation-form is another option, as the launching pad is already supplied in the original theme selected for decoration and development. This has been one of the most consistently used forms throughout keyboard history, from the Elizabethan virginalists through to the present day. Another possibility is the transcription, when an existing work is reformatted, usually for another instrument. This can be straightforward and faithful, such as Liszt's piano version of Berlioz's Symphonie Fantastique; or it can be highly elaborated for virtuosic or whimsical purposes, such as the luridly decadent Invitation to the Dance, in which Godowsky escorts Weber through the most perfumed of labyrinths. (Coincidentally, Berlioz made an orchestral transcription of this same Weber piece.)

When I had little time or confidence to write my own music, I had lots of fun transcribing that of others. This skill has survived among pianists, especially in recent years, and many examples are first-rate. Supplying cadenzas for Mozart concertos is another survivor - although where pianists in the past would almost always supply their own cadenzas, written-down or improvised, and these would be in a contemporary style (as with Beethoven, Brahms, Busoni and Schnabel), today one is more likely to hear pastiche-Mozart, and even these are often composed by someone else.

A commission, however informal, is a further encouragement to get to work. My cello concerto (The Loneliest Wilderness) came about because a bassoonist asked me to write him a piece with orchestra. I was challenged and delighted, but as I was writing the piece, I began to realise that it would sound better on the cello. Pieces written as gifts are yet another avenue. I was visiting a friend to celebrate his birthday and had no idea what to buy him, so, on the plane journey, I wrote him a small waltz based on the letters of his name. Out of this came my Suite R-B - six similar little pieces. A surprise performance of my Advent Calendar carol by Westminster Cathedral Choir led to them asking me to write them a mass. I'm finishing the final movement at the moment.

The decisions a composer makes when choosing the place to write that slur or accent are not arrived at arbitrarily; and structural issues or harmonic progressions can be wrestled with for months, with solutions only arrived at after much anguish. When we have had to struggle with similar questions in our own composing, I think we get a greater understanding of the music we play by others. The migration of pianist-composers over the past half century is not a healthy development. Certain musical muscles have been allowed to atrophy, and, apart from anything else, we have missed out on some fun - not to mention some royalties!

· Stephen Hough conducts the world premiere of his cello concerto with Steven Isserlis and the RLPO on March 17 at the Philharmonic Hall, Liverpool. Box office: 0151-709 3789.